1. Field of the Invention
The present invention relates to a light emitting device and to a display device. In addition, the present invention relates to electronic equipment in which the light emitting device or the display device is mounted. The term light emitting device as used in this specification indicates devices that utilize light emitted from a light emitting element. Examples of the light emitting elements include organic light emitting diode (OLED) elements, inorganic material light emitting diode elements, field emission light emitting elements (FED elements) and the like. The term display device as used in this specification indicates devices in which a plurality of pixels are arranged in a matrix shape, and image information is visually transmitted, namely displays.
2. Description of the Related Art
The importance of display devices that perform display of images and pictures has continued to increase in recent years. Due to their advantages such as high image quality, thin size, and light weight, liquid crystal display devices that perform display of an image by using liquid crystal elements are widely utilized in various types of display devices, such as portable telephones and personal computers.
On the other hand, the development of display devices and light emitting devices that use light emitting elements is also advancing. Elements that use many different types of materials over a wide-ranging area, such as organic materials, inorganic materials, thin film materials, bulk materials, and dispersed materials exist as light emitting elements.
Organic light emitting diodes (OLEDs) are typical light emitting elements currently seen as promising for all types of display devices. OLED display devices that use OLED elements as light emitting elements are thinner and lighter than existing liquid crystal display devices, and in addition, have characteristics such as high response speed suitable for dynamic image display, a wide angle of view, and low voltage drive. A wide variety of applications are therefore anticipated, from portable telephones and portable information terminals (PDAs) to televisions, monitors, and the like. OLED display devices are under the spotlight as next generation displays.
In particular, active matrix (AM) OLED display devices are capable of high resolution (large number of pixels), high definition (fine pitch), and a large screen display, all of which are difficult for passive matrix (PM) type displays. In addition, AM-OLED display devices have high reliability at lower electric power consumption operation than that of passive matrix OLEDs, and there are very strong expectations that they will be put into practical use.
OLED elements are structured by an anode, a cathode, and an organic compound containing layer sandwiched between the anode and the cathode. Normally the brightness of light emitted from the OLED element is roughly proportional to the amount of electric current flowing in the OLED element. A driver transistor that controls the light emission brightness of a pixel OLED element is inserted in series with the OLED element in AM-OLED display device pixels.
Voltage input methods and current input methods exist as driving methods for displaying images in AM-OLED display devices. The voltage input method is a method in which a voltage value data video signal is input to the pixels as an input video signal. On the other hand, the current input method is a method in which a current value video signal is input to the pixels as an input video signal.
The video signal voltage is normally applied directly to a gate electrode of a pixel driver transistor in the voltage input method. If there is dispersion, not uniformity, in the electrical characteristics of the driver transistors across each of the pixels when the OLED elements emit light at a constant current, then dispersion will develop in the OLED element driver current of each of the pixels. Dispersion in the OLED element driver current becomes dispersion in the brightness of light emitted from the OLED elements. Dispersion in the brightness of light emitted by the OLED elements reduces the quality of the displayed image as a sandstorm state or carpet-like pattern unevenness is seen over an entire screen. Stripe shape unevenness is also found, depending upon the manufacturing process.
In particular, a relatively large electric current is necessary in order to obtain a sufficiently high brightness when OLED elements presently capable of being used, which have low light emission efficiency, are applied as a light emitting device. As a result, it is difficult to use amorphous silicon thin film transistors (TFTs), which have low current capacity, as the driver transistors. Polycrystalline silicon (polysilicon) TFTs are therefore used as the driver transistors. However, there is a problem with polysilicon in that dispersions in the TFT electrical characteristics are likely to develop due to causes such as faults in the crystal grain boundaries.
The current input method can be used as one effective means in order to prevent dispersion in the OLED element driver current that occurs in this type of voltage input method. A video signal data current value is normally stored with the current input method, and an electric current identical to, or several times as large as, the value of the stored electric current (positive real number multiples, including those less than 1) is supplied as the OLED element driver current.
A typical known example of a pixel circuit of a current input method AM-OLED display device is shown in FIG. 10A (refer to Non-Patent Document 1). Reference numeral 516 denotes an OLED element. This pixel circuit uses a current mirror circuit. Video signal data current values can be accurately stored as long as two transistors structuring the current mirror have identical electrical characteristics. Even if there is dispersion in the electrical characteristics of the driver transistors of different pixels, dispersion in the brightness of light emitted by the OLED elements can be prevented as long as the two transistors within the same pixel each have identical electrical characteristics.
Another typical known example of a pixel circuit of a current input method AM-OLED display device is shown in FIG. 10B (refer to Non-Patent Document 2). Reference numeral 611 denotes an OLED element. This pixel circuit has a short circuit between a drain electrode, and a gate electrode, of a driver transistor itself when a voltage corresponding to a video signal is written into the gate electrode of the driver transistor. A video signal data current is made to flow in this state, and the gate electrode is then electrically insulated. By doing so, an electric current having a value identical to the data current during write-in is supplied to the OLED element by the driver transistors, provided that the driver transistors are operated in the saturated region. Dispersion in the brightness of light emitted by the OLED elements can therefore be prevented, even if dispersion exists in the electrical characteristics of the driver transistors of each pixel.    [Non-Patent Document 1] Yumoto, A., et al., Proc. Asia Display/IDW '01, pp. 1395-1398 (2001).    [Not-Patent Document 2] Hunter, I. M., et al., Proc. AM-LCD 2000, pp. 249-252 (2000).
The data current value should be able to be accurately stored with FIGS. 10A and 10B, as discussed above, but there are serious problems as stated below.
First, a problem with the pixel circuit of FIG. 10A is that there is a precondiction in which the two transistors 512 and 513 that structure the current mirror must have identical electrical characteristics. Provided that it is considered during design, it is possible to manufacture both transistors adjacently on a substrate, and dispersion can be reduced to a certain extent. However, dispersions in the electrical characteristics of TFTs, such as threshold voltage and field effect mobility, that exceed a permissible limit normally remain in present-day polysilicon due to causes such as faults in the crystal grain boundaries.
Specifically, it becomes necessary to keep brightness within a range on the order of 1%, for example, if a 64 gray scale image is displayed. However, storing the data current values at a precision of 1% with the pixel circuit of FIG. 10A is difficult to achieve with the polysilicon normally in use at present. In other words, a sufficiently uniform, high quality display image over an entire screen. without irregularities, cannot be obtained by only using the pixel circuit of FIG. 10A.
Next, the fact that the video signal data current written into the pixel has the identical value to the OLED element driver current when the OLED element emits light is a problem with the pixel circuit of FIG. 10B. The fact that both electric currents must have identical values is a very severe restriction in practice when manufacturing an AM-OLED display device.
Specifically, a large amount of parasitic capacitance and parasitic resistance exists in signal lines and the like in an actual AM-OLED display device. As a result, it often becomes necessary to take steps to make the video signal data current larger than the OLED element driver current. In particular, it becomes extremely difficult to write in the video signal data current of dark portions for cases in which the video signal data current is made into an analog value for gray scale expression.